Photosynthesis

Photosynthesis is a chemical process which occurs in green-leafed plants. It converts carbon dioxide and water into sugar in the form of starch, using the energy of solar radiation. Photosynthesis is the gateway for nearly all energy in the various cycles of life on the planet.

History of the discovery of photosynthesis

People used to believe that plants grew by 'eating soil'. Although plants obtain various essential nutrients from soil, and nitrogen from nitrobacteria in the soil, and water from the soil is absorbed by the roots, the soil itself is not a part of plant growth.

Do plants eat soil?

The first person to actually test the hypothesis that plants ate soil as a Dutchman, Jan Baptist van Helmont (1580 - 1644). He was an early adopter of empirical science. This 'new idea' was that knowledge could not be gained just by reading what people had previously believed, but by carrying out experiments to test ideas. Helmont wanted to know if plants ate soil, or obtained their weight from some other source, such as water. To do this he grew a willow tree under controlled conditions. He measured the weights of an amount of dry soil and a willow tree sapling. He placed the soil in container, and planted the young tree in it. After five years, he measured the weight of the tree and the weight of the soil again.

Unsurprisingly, the tree had grown, putting on 74 kg, but the soil had not changed mass very much at all. Since Helmont had watered the plant throughout the experiment, he concluded that the plant had grown because of the mass of the water. A logical but false conclusion.

Why do plants need light?

Joseph Priestly, the famous English chemist, published his work on the discovery of oxygen in 1771. He observed that a candle inside a bell jar would consume part of the air, and the candle would extinguish. By placing a leaf of mint in the jar in sunlight, the candle would stay lit longer. The leaf was producing the gas which fueled the flame. Priestley had discovered oxygen.

Jan Ingenhousz (1730 - 1799), was a Dutch physician who made a stunning discovery: plants in water in bright sunshine release tiny bubbles of oxygen. Without realising its full significance to science, he had discovered the most important chemical reaction in life: photosynthesis.

In 1779, Ingenhousz began to tinker with leaves in jars of water, he knew the bubbles he was observing were oxygen, and tested this idea by re-igniting a smouldering splint as Priestly had explained. He also went on to discover that plants give off a different gas in the dark - this gas was carbon dioxide.

Ingenhousz had discovered that plants do not eat water only, but also air!

Why are plants green?

The next question botanists wanted to answer was how plants grew from the combination of carbon dioxide, water, and light. To do this, they needed more powerful microscopes, and a genius. The genius was Julius von Sachs (1832 - 1897), a German working at he University of Würzburg.

Among his many discoveries, Sachs found that the cells of leaves contained chloroplasts. When chloroplasts are exposed to sunlight, they produce glucose (sugars) in the form of starch. It is this starch which is the material from which plants grow. By careful measurement, Sachs was able to determine that the proportions of the substances involved in photosynthesis followed this equation:

$$6CO_2 + 6H_2O →↖{\text light} C_6H_{12}O_6 + 6O_2$$

but he did not have the equipment to understand the details of how planets did this. The techniques needed to discover the details of photosynthesis would not be available for nearly a century.

How does a plant grow from thin air?

The mechanism by which plants grow was not understood until the 1950s, when Andrew Benson, James Bassham, and Melvin Calvin, working in California, tracked individual radioactive carbon atoms as they passed from the air, through fixing by the chloroplasts, to become part of the plant by means of a complicated process involving a 'battery' for storing energy: the ATP - ADP molecule.

This mechanism is called the Calvin-Benson-Bassham Cycle, and it is a very complicated process. There are three basic phases: carbon fixation, reduction, and regeneration of ribulose.

ATP and chemical energy

ATP is short for adenosine triphosphate. The molecule consists of adenine, ribose (a 5-carbon sugar), and three phosphate groups.

ADP is short for a very similar molecule called adenosine diphosphate. Like ATP, ADP has an adenine and ribose part, but is different in having only 2 phosphate groups.

It takes energy to add a third phosphate to ADP to form ATP. When this third phosphate is removed from ATP to form ADP, the energy is recovered. This makes ATP a battery - storing energy for processes in cells. These processes which use ATP energy include active transport across cell membranes, protein synthesis, and muscle contracton.

Autotrophs and Heterotrophs

What makes the photosynthesis reaction so useful for life is that plants need carbon dioxide, and release oxygen as a waste product. Animals breathe oxygen, and breathe out carbon dioxide. The perfect deal!

Solar Radiation

The Sun is the supplier of practically all the energy of the biosphere and the physical forces which operate in the troposphere, where life is found. The only source of energy that is not solar is geothermal - the heat of uranium and other radioactive elements, whose nuclear decay in the Earth's interior heats the planet. But this contribution to the biosphere is minor - more than 99% of the energy comes from the Sun.

Even fossil fuels, oil, coal and methane gas, are ultimately solar energy. They are the residues of long-dead organic matter, which was created as a primary or secondary product of photosynthesis, using the Sun's energy to combine CO2 and water.

Incidence and Absorption

Re-radiation

Solar and Earth radiation spectra: the Sun's incident radiation is short-wavelength, and the Earth's is long-wavelength

A black-body, or perfect radiator, emits radiation, such as infrared (heat), at a wavelength inversely proportional to surface temperature (absolute temperature, K): Wien's Displacement Law

$$λ ∝ 1/T$$

The rate of emission (power) of the radiation from a black-body is directly proportional to the fourth power of the temperature: Stefan–Boltzmann law

$$P ∝ T^4$$

This means that the cooler the surface, the longer the wavelength of radiation it emits. That is why we can see the Sun, but at night we do not see the Earth, but can feel the heat coming from hot asphalt.

The Sun's radiation incident to the surface of the Earth is mainly between 0.2 and 4.0 µm (short-wave radiation), while the Earth re-radiates infrared between 4 and 25 µm (long-wave radiation).

The Energy Balance

If the amount of energy reaching the upper layers of the Earth's atmosphere is 100%, then 30% is reflected (albedo), 20% is absorbed by clouds and the atmosphere, and 50% reaches the surface.

On a long-term average, the same amount of energy will be re-radiated back into space, either directly or via atmospheric absorption/re-radiation.